1. Field of the Invention
This invention relates to communication systems and particularly to such systems for use in industrial control applications. More particularly, the invention relates to a dual-ring configured, network communication system that continues uninterrupted communication even in the event of a failure in communication lines.
2. Background
According to certain dual channel communication methods and systems, information is sent twice over respective communication channels. The information is in data packets. If one of the communication channels breaks, the information is still delivered on the remaining channel. (Channels also referred to herein as “paths.”) Dual channel communication is used in situations where it is critical that information be delivered within a certain time frame in spite of limited communication path failure. In such critical situations, another alternative is to propagate communication twice in respective directions in a single, ring-configured communication channel. In this manner, if communication in one direction fails, information is still delivered in the other direction around the ring.
Both dual channel and single-ring type systems work well when only one of two transmission paths fails. However, some users demand a network that continues to deliver communication despite multiple transmission path failures. One way to meet this demand is to configure dual communication channels into respective rings. However, this may be problematic when using a communication protocol that requires duplicate data packets to arrive at the same node in separate communication paths within a predetermined time interval relative to one another. An example is the ControlNet standard, which was defined by Allen Bradley and is now supported by Rockwell Automation, for example.
(“ControlNet” is a trademark of ControlNet International Ltd.) In particular, ControlNet requires packets in respective communication channels to arrive at the same destination ControlNet node within 1.6 μs of each other. This application hereby incorporates by reference ControlNet International Specification 2.0 in its entirety.